Interest Rate Negotiation Calculator

It reports a large negative number rather than 0 or a warning. Interest Saved only checks that both simulations' total-interest figures are non-negative before subtracting (lines 44-46), but when a payment cannot cover a month's interest, the engine's sentinel result sets that side's total interest to exactly 0 rather than leaving it undefined (lines 17-18) — so the subtraction still runs and produces a misleadingly large negative "savings" figure.

No — Months Saved uses a stricter guard than Interest Saved. It checks that both Current Payoff Time and Reduced Payoff Time are non-negative before subtracting them (lines 49-52), and the sentinel value for a payment too small to cover interest is exactly -1, which fails that check. In that situation Months Saved correctly falls back to 0 instead of reporting a real number.

No — each simulation only receives its own rate. Current Payoff Time is computed by running the shared payoff simulation with Current APR and never sees Negotiated APR at all (line 41), so nudging Negotiated APR by any amount leaves Current Payoff Time completely unchanged. The same independence holds in reverse for Reduced Payoff Time and Current APR.

Neither one clearly dominates — Current APR and Balance move Interest Saved by strikingly similar ±10% spans, about 68% and 65% of Interest Saved respectively, well ahead of Negotiated APR's roughly 23% and Monthly Payment's roughly 46%. Both the size of the debt and how high the current rate is matter almost equally to how much a rate cut is worth.

At the default $10,000 balance and 24.9% Current APR, Monthly Payment needs to stay above about $207.50 for the current-rate simulation to run normally; below that, the payment cannot cover a month's own interest and the simulation returns its -1 sentinel instead of a real payoff time. The Negotiated APR side has a lower floor, about $125 a month at the default 15% rate, since a lower rate accrues less interest to cover in the first place.